During the campaign, Donald Trump railed against "sanctuary cities" — generally understood to be jurisdictions where local law enforcement doesn't cooperate sufficiently with federal immigration authorities. Sanctuary cities were an especially hot issue because of the death of Kate Steinle, a tourist shot by a Mexican national in San Francisco in 2015.

In an August campaign speech, Trump promised to "end" sanctuary cities by blocking their federal funding. But keeping that promise will be complicated by the fact that the term "sanctuary city" has no clear legal meaning. It comes out of the 1980s "sanctuary" movement of Christian churches that sheltered refugees from Central America, but these days the term is used increasingly by groups opposed to illegal immigration. The Center for Immigration Studies, for example, has a map tracking "sanctuary" jurisdictions which "threaten public safety."

"There is no definition of a sanctuary city," says Lena Graber, an attorney at the Immigrant Legal Resource Center in San Francisco. She says different places have taken different approaches. "There are a lot of sanctuary policies that are more just about not asking about immigration status by city agencies or law enforcement," she says, while other jurisdictions are called "sanctuaries" because their jails won't hold people at the request of Immigration and Customs Enforcement.

In the latter case, the jails that don't honor ICE "detainer" requests often do it for reasons that are practical, not political.

The jail in Lafayette Parish, La., for instance, stopped honoring ICE detainers out of a fear of lawsuits by people who might be able to claim they're being held illegally.

"We're depriving them of their libery," says Sheriff Mark Garber, "on a suspicion that hadn't been investigated promptly or acted on at all."

Sheriff Garber says when his predecessor realized the federal government wouldn't indemnify the parish in case of lawsuits, he stopped honoring ICE detainer requests. But that led to accusations that the parish had become a "sanctuary," especially during the last sheriff's election. So when Garber took over, he instituted a compromise policy: the Lafeyette jail will hold people, but for no longer than 48 hours.

"I'm still taking a risk to cooperate with the federal government, because they're not offering me any reciprocal protection," Garber says. "I still think we're exposed, but I'm choosing as the leader in this community to go ahead and draw the bright line at the 48 hours."

Those bright lines are drawn differently in almost every jurisdiction. The King County jail in downtown Seattle, for instance, has a policy of honoring only the ICE detainer requests that come with an order from a judge. Other places take the severity of the person's criminal record into account.

Broadly speaking, the attitude in "sanctuary cities" is that local law enforcement shouldn't help to deport people accused of minor offenses, but "serious criminals" are less deserving of that protection.

"The use of local law enforcement for general deportation reasons, for low-level offenders, is not appropriate," said Los Angeles Police Chief Charlie Beck earlier this week. But he added, "We already, when somebody commits a serious violent crime, cooperate to what extent is legal with [the Department of Homeland Security]."

King County Sheriff John Urquhart makes a similar distinction. His deputies aren't allowed to ask people about their immigration status, and he says when they arrest someone for a minor crime, that shouldn't lead to deportation.

"Perhaps he's a shoplifter, stealing baby food to feed his family," Urquhart says. "I don't think we should be involved in turning those people over to Immigration."

But Urquhart adds, "That doesn't mean we can't deal with the true criminals." He gives the example of detectives investigating foreign-born drug dealers. "We want to let ICE know that they may want to take a look at this person, because they're a criminal, and I want them out of this country just as much as Donald Trump does."

It's a distinction the public generally agrees with. But it doesn't sit well with Thomas Saenz, president and general counsel of the Mexican American Legal Defense and Educational Fund.

"If a sheriff or sheriff's deputy or anyone else is, without a conviction, deciding this is a bad person, and [says], 'We want to find some other means of getting them out of the community and therefore we're going to call ICE,' that's bad policy. In fact, it is arguably unconstitutional policy," Saenz says.

"There should be a strict, strict wall of no cooperation between local law enforcement and federal immigration authorities," he says.

That kind of wall seems absurd to Sheriff Chuck Jenkins, of Frederick County, Md. He blames that kind of attitude for the fact that millions of "criminal aliens," as he puts it, are still at large in the U.S.

"Our failures to enforce the immigration laws as they are written, I think largely falls back to local law enforcement's failure to cooperate," Jenkins says.

He would like to see more jurisdictions do what his county does. His deputies aren't allowed to ask people about immigration status during contacts on the street, but that changes once a suspect is booked into jail.

"We ask two basic questions of everybody: 'Where were you born?' and 'What country are you a citizen of?' Any answer other than 'the United States of America,' we do an immigration status check," he says. If his deputies get an indication the person is in the country illegally, they hold him for ICE.

Jenkins is a vocal opponent of the "sanctuary city" idea, but he also acknowledges that sanctuary policies vary from place to place. Still, he doesn't think the Trump administration will have trouble identifying which jurisdictions deserve the label.

"I think the line is very clear in the fact that if your county does nothing — nothing — to cooperate, takes no action to cooperate with ICE, I think that by the very fact that you don't do anything you're a sanctuary city."

But if the bar is that low — a complete lack of cooperation with ICE — then the actual list of sanctuary cities may turn out to be quite short.

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SCOTT SIMON, HOST:

During the campaign, Donald Trump railed against cities where law enforcement limits its cooperation with federal immigration enforcement. Speech in August, he pledged to punish places like that by cutting their federal funding.

(SOUNDBITE OF ARCHIVED RECORDING)

DONALD TRUMP: We will end the sanctuary cities that have resulted in so many needless deaths.

SIMON: But as NPR's Martin Kaste reports, keeping that promise will be difficult given that there's no agreement on what the term sanctuary city even means.

MARTIN KASTE, BYLINE: This week, officials in liberal cities like Seattle and Chicago were sounding defiant, saying Trump's election wouldn't change their policies on how to handle people who are in the country illegally. Here's Los Angeles Police Chief Charlie Beck.

CHARLIE BECK: The use of local law enforcement for general deportation reasons for low-level offenders is not appropriate, and we don't do that.

KASTE: But notice, Beck specifies low-level offenders. When it comes to people who've committed more serious, violent crimes, he says the LAPD does cooperate with the feds. The truth is, jurisdictions with a sanctuary label are sanctuaries only up to a point, and that point varies a lot from place to place.

JOHN URQUHART: There is not a real clear definition of what a sanctuary city is. It's almost a political designation that one side or the other places on a city or, in some cases, a county.

KASTE: That's John Urquhart, the sheriff in King County, which includes Seattle. His department's policy forbids deputies from asking people in the street about their immigration status, and he thinks that's as it should be.

URQUHART: That doesn't mean we can't deal with the true criminals that we come in contact, that we arrest, that we put in jail, that we want to let ICE know they may want to take a look at this person because they're a criminal, and I want them out of the country just as much as Donald Trump does.

KASTE: As an example, he says if his deputies were investigating a drug-runner that they thought was in the country illegally, they should feel free to tip off immigration. And this kind of gray area is common in these liberal jurisdictions, the idea that sanctuary protects the little guy but isn't meant to shield serious criminals. It's a distinction the public tends to agree with, but Thomas Saenz doesn't like it.

THOMAS SAENZ: I think that that's an approach that ignores due process.

KASTE: Saenz is president and general counsel of the Mexican-American Legal Defense and Educational Fund. He says when police make these distinctions, it opens the door to abuse.

SAENZ: If a sheriff or a sheriff's deputy or anyone else is without, you know, a conviction deciding this is a bad person, and we want to find some other means of getting them out of the community and therefore we're going to call ICE, that's bad policy. In fact, it is arguably unconstitutional policy.

KASTE: He'd like to see what he calls a strict, strict wall of no cooperation between local law enforcement and immigration authorities. That's an absurd idea to Sheriff Chuck Jenkins of Frederick County, Md. He says that kind of thinking is why we still have millions of people in this country illegally.

CHUCK JENKINS: Our failures to enforce the immigration laws as they are written, I think, largely it falls by to local law enforcement's failure to cooperate.

KASTE: The thing is, the local's decision whether to cooperate is not always about politics. For instance, some jails no longer hold people at the request of immigration but not because they want to be sanctuaries. It's because they're afraid of being sued for holding people illegally. The Trump administration may have a challenge trying to decide which of these places are true sanctuary cities. But Sheriff Jenkins doesn't think it'll be that hard.

JENKINS: I think the line is very clear in the fact that if your county does nothing - nothing to cooperate, takes no action to cooperate with ICE, I think that by the very fact you don't do anything, you're a sanctuary city.

KASTE: But if the bar is that low, doing nothing to cooperate, then the list of places to punish for being sanctuary cities could end up being very short.